(1) Field of the Invention
The present invention relates to a semiconductor integrated circuit device, and more particularly to a radio-frequency integrated circuit device having an adjustable matching circuit.
(2) Description of the Related Art
A conventional short-circuit stub which is used in a semiconductor integrated circuit device and to which a bias voltage is applied is configured as shown, for example, in FIGS. 1A and 1B. FIG. 1B shows this in an equivalent circuit diagram. A signal line 1 is short-circuited to ground 3 at an RF short-circuit point 6 through a capacitor 2, while it is supplied with a direct current (DC) voltage from a biasing point 4. That is, the capacitor 2 operates to effect RF short-circuiting and, at the same time, serves to block a direct current component.
The actual parameters of elements within the fabricated integrated circuit result in values different from those originally designed due to distributed capacitance or distributed inductance. Thus, there occur cases such integrated circuit lacks optimum properties. There have been attempts to make the circuit adjustment by adjusting a matching circuit after the fabrication of the integrated circuit.
In a conventional micro-strip line matching circuit shown as an example in FIG. 2, a micro-strip transmission line 10 and island-like metal lands 11 disposed adjacent thereto are interconnected by bonding wires 12, thus effecting changes in impedance and making appropriate adjustments in the electric characteristics. This functions in the same way as in the case wherein open stubs are provided in a matching circuit, and can be used irrespective of whether a bias voltage is applied or not.
An example wherein an adjustable matching circuit having a chip capacitor and a microstrip line is formed on a dielectric substrate has been disclosed in Japanese Patent Application Kokai Publication No. Sho 63-224503. In this example, there is involved an operation of moving a chip capacitor provided to interconnect the two lines disposed in parallel so that the position at which the signals pass through the chip capacitor is changed. In this case, there develops a change also in the impedance of the circuit comprising the two lines and the chip capacitor. In this example, the chip capacitor plays a role of blocking DC components and changing the impedance of the matching circuit.
In the prior art, it was difficult to provide integrally on the same substrate an adjustable matching circuit having a short-circuit stub to which a bias voltage is applied. Especially, in the case of a matching circuit constituted by coplanar lines, a problem was that, since the lines are surrounded by ground potential surface, the method of interconnecting the island-like lands as explained above was not suitable. Also, in the method wherein a chip capacitor is used, since it is on the premise that an external component is to be incorporated in the circuit, this has presented a problem in the enhancement of integration.